Juat a FYI though. DC to Cumberland isn't a "Rails to Trails". It's basically a 2 track tow path. WMRT, and the GAP *are* Rauils to Trails, and there is a night and day difference. C&O is best on a Cyclocross bike if you're looking for speed, a MTB or Hybrid if you're looking for comfort. No way I'd do the C&O on a road bike.
-Roger